Pre-filled hypodermic syringes provide an exact dose of a desired medication or other fluid ready for immediate use. Consequently, there exist many situations where pre-filled hypodermic syringes are preferable over conventional syringes that are filled when needed. One advantage of pre-filled hypodermic syringes is that the dosage of medication or fluid contained within the syringe is factory controlled. As such, variations in mixture concentrations or dosages that can occur each time an empty syringe is manually filled can be eliminated. Additionally, by pre-filling a syringe at the point of manufacture, the needle of the pre-filled syringe can remain sterile until its moment of use. With conventional empty syringes the syringe must be handled as fluid is drawn into the syringe, thereby resulting in the increased risks of contaminating either the fluid and/or the needle of the syringe.
One of the largest advantages of the pre-filled hypodermic syringes is that of convenience. Pre-filled hypodermic syringes eliminate the need to fill conventional empty syringes from vials, thereby saving both time and labor. Furthermore, the use of pre-filled hypodermic syringes enables people unskilled in mixing solutions and filling syringes to properly administer intravenous injections. As a result of the ease of using pre-filled hypodermic syringes, pre-filled hypodermic syringes have become commonplace for use in high volume situations. For instance, pre-filled hypodermic syringes are commonly used during immunization programs where exacting dosages of vaccine are administered to a large number of people by only a few attending medical personnel. Similarly, pre-filled syringes are commonly used by people who require repeat self-administered injections such as diabetics and hemophiliacs. The use of pre-filled syringes provides such persons with the convenience of not having to fill a syringe each time an injection is required. Furthermore, pre-filled hypodermic syringes permit an untrained person to administer a proper dosage of medication to a person, such as a diabetic, should that person be unable to administer such an injection him or herself.
Conventional hypodermic syringes, be they pre-filled or not, work essentially in the same manner. The syringe contains a fluid reservoir coupled to a hollow cannula. By engaging the plunger of the hypodermic syringe, the volume of the fluid reservoir can be changed and fluid can be either discharged from, or drawn into, the syringe through the lumen of the cannula. In pre-filled syringes, the syringe plunger is drawn to a preset position thereby retaining a desired volume of fluid within the syringe. The lumen of the cannula is selectively blocked, thereby preventing the desired volume of fluid within the syringe from changing. In conventional prior art pre-filled hypodermic syringes, the lumen of the cannula is blocked by positioning the sharpened point of the cannula within an elastomeric stopper. The elastomeric stopper envelops the sharpened point of the cannula in a fluid impervious manner thereby preventing the flow of fluid out of the syringe through the cannula. Such elastomeric stoppers are often formed as part of the protective sheathing for the syringe cannula as exemplified by copending U.S. patent application Ser. No. 07/804,091 entitled DISPOSABLE PRE-FILLED SYRINGE WITH RETRACTABLE NEEDLE and assigned to Medtech Group, Inc. to assignee herein.
The use of elastomeric stoppers is commonplace because they are effective, easy to use and inexpensive to manufacture. However, in certain situations elastomeric stoppers do have disadvantages. Since elastomeric stoppers block the lumen of a syringes cannula at its sharpened point, the fluid contained within the syringe also fills the lumen of the cannula leading to the elastomeric stopper. In many hypodermic syringes the lumen within the cannula is extremely narrow. As such, with certain solutions, the presence of the fluid within the confined space of the cannula lumen promotes crystallization. The resulting crystallization could then obstruct the lumen.
It is therefore a primary objective of the present invention to provide a pre-filled syringe wherein the fluid within the syringe is held in a manner that prevents the loss of fluid prior to use, and similarly does not permit the retained fluid to enter the cannula of the syringe until the syringe is enabled and readied for use.